1. Field of the Invention
The present invention generally relates to voltage sources for electronic devices, particularly low-power voltage sources for analog circuits, and more specifically to a circuit for providing a low-voltage reference which is highly temperature stable (i.e., has low effective temperature coefficients) and supply voltage stable.
2. Description of Related Art
Modern electronic devices employ a wide variety of integrated circuits to carry out various logic functions, including simple designs for, e.g., wristwatches, and more complicated designs for, e.g., data processing systems. An integrated circuit (IC) is essentially a multitude of interconnected circuit elements, such as logic gates, amplifiers, inverters, etc., which are formed of electrical components such as transistors, diodes, resistors, etc. These components are miniaturized, and fabricated on a common substrate.
The substrate of an integrated circuit is formed of a semiconductor material, such as silicon or germanium, which has been doped (mixed with certain impurities) in a pattern to create solid state elements, such as complementary metal-oxide semiconducting (CMOS) devices. An IC can be single layer, with interconnecting leads formed on the substrate between circuit elements, or multi-layer, with interconnecting vias formed between adjacent and non-adjacent layers.
A primary challenge in designing precision integrated circuits is to control circuit parameters, such as bias currents, in view of temperature variations and supply variations. During the design of integrated circuits, anticipating and controlling operating fluctuations due to changes in environmental conditions such as temperature, requires a complicated analysis. Temperature sensitivity, fabrication variations and supply voltage variations exhibit complex relationships among each other. In integrated circuits, precision oscillators (more particularly, oscillators with feedback) are designed to comply with exacting specifications that have little margin for deviation. Surmounting these tight tolerances over a wide range of temperatures is very challenging for a designer. For such oscillators, these difficulties are exacerbated by today's higher frequency requirements. Precision circuits which are very sensitive to temperature variations include voltage-controlled oscillators, which are used in phase-lock loop circuits that provide clock signals to, e.g., high-speed data processing systems.
A variety of techniques are known for stabilization of a circuit over a temperature range which the circuit may endure. For example, temperature variations in a voltage-controlled oscillator can be significantly reduced by temperature-independent biasing techniques. One problem with this approach, however, relates to the use of lower power voltage supplies for the reference circuits.
In general, as CMOS technology continues to evolve to lower supply voltages, most existing analog circuits designed for higher supply voltages will fail to operate due to the large degree of device stacking. Reference circuits in particular, such as thermal voltage (V.sub.therm) or bandgap generators, are often composed of four levels of stacking, and have poor characteristics below a power supply (V.sub.dd) of about 1.5 volts. FIG. 1 shows a common V.sub.therm -referenced current source. Transistors Q.sub.1 and Q.sub.2 have areas that differ by a factor of n, and the feedback loop formed by CMOS devices M.sub.1, M.sub.2, M.sub.3, and M.sub.4 forces these two transistors to operate at the same bias current. As a result, the circuit's output current I.sub.out is equal to [V.sub.therm *ln(n)]/R, where R is the resistance of resistor R, and V.sub.therm is kT/q (k is Boltzmann's constant, T is absolute temperature, and q is electron charge). It turns out that both R and V.sub.therm have positive temperature coefficients (TCs), which yield a relatively temperature-independent output current for supply voltages above 1.5 volts; however, the four levels of stacking (M.sub.4, M.sub.2, R, Q.sub.2) make it unsuitable for lower voltages.
Another temperature-independent biasing technique uses base-to-emitter voltage (V.sub.be) referencing, as shown in FIG. 2. In this design, the same feedback loop is present, but the absence of a second transistor makes the output current I.sub.out equal to V.sub.be /R, where V.sub.be is the base-to-emitter voltage of transistor Q.sub.1. This approach may be implemented with 3-high stacking, but it still has a large overall negative TC due to the negative TC of V.sub.be, and to the large positive TC of diffused and polysilicon resistors.
Bandgap generators use a weighted combination of V.sub.therm and V.sub.be generators to produce very low temperature dependence, as seen in FIG. 3. The circuit's output current I.sub.out is equal to V.sub.o /R.sub.2, where R.sub.2 is the resistance of resistor R.sub.2 and V.sub.o (the output voltage) is equal to V.sub.be +xV.sub.therm ln(n). The parameter "x" (the multiplier for the resistor connected to the input of the operational amplifier) represents the weighting of the V.sub.therm -dependent portion of the output voltage. While this design can thus be tuned to substantially reduce overall temperature dependence, it still suffers at low supply voltages (V.sub.dd), from the large degree of device stacking in the V.sub.therm generator (four levels of stacking).
In light of the foregoing, it would be desirable to devise a voltage reference circuit for low V.sub.dd applications which has an improved effective temperature coefficient. It would be further advantageous if the circuit could be used in high-speed oscillator applications.